(1) Field of the Invention
The present invention relates to a battery capacity measurements, and more particularly to the battery capacity measurements by battery internal resistance of a constant discharge circuit.
(2) Description of the Prior Art
The battery may be the power source for all portable electronic devices. For example, mobile phones, laptop computers, personal digital assistants, music players, etc. They are all depends on battery power. Portable electronic devices nowadays adopt rechargeable batteries to recover the power loss consumed by the portable electronic devices.
The aging of the rechargeable battery is most commonly detected by the internal resistance changes of the battery. The internal resistance of the rechargeable battery is small just when the rechargeable battery was produced from a factory. But after a long period time of charging and discharging of the battery, the internal resistance gradually increased until that the resistance is too large for the battery to deliver power to electronic device. Most of the aging of the battery are caused by the increase of the internal resistance.
A rechargeable battery with excellent charge and discharge management can repeatedly be used for hundreds of times, even for thousands of times before aging.
The application of rechargeable Li-ion battery is ubiquitous in recent years. Although the Li-ion battery has many advantages, all the Li-ion battery has a property that the battery will fail for battery's over-charging and under-discharging. The over-charging or short circuit can cause the battery temperature to raise and thus damage the battery structure, or finally the battery may explode. When the voltage of the Li-ion battery is increased to 4.30±0.05V during charging, we should immediately stop charging to avoid overcharging or explosion; and when the voltage of the Li-ion battery is decreased to under 2.3±0.1V during discharging, we should immediately stop the discharge, so as not to damage the battery's cycle life.
The supply voltage VBAT between the positive electrode and the negative electrode of the battery is not equal to the battery open circuit voltage VOC. This phenomenon can be explained by an internal resistance RINT of the battery in the following equation:VBAT=VOC+I*RINT,
wherein I is the current passing through the battery.
A typical battery internal resistance RINT is about 100 milli-ohm. Thus, when the charging current I is 1 Ampere and the battery open circuit voltage VOC is 4.1V, the battery supply voltage VBAT is 4.2V. On the other side, when the discharging current I is −1 A and the battery open circuit voltage VOC is 4.1V, the battery supply voltage VBAT is 4.0V.
Therefore, if we did not take the internal resistance R into consideration, we will under-charging the battery or over-discharging the battery. Furthermore, we can not calculate the accurate battery capacity without the internal resistance RINT.
Consequently, how to accurately measure the gradually increasing value of the internal resistance RINT after a long period of charging and discharging cycles becomes an essential factor for battery management chip design. However, in some previously published techniques, there are still rooms for improvement on simplicity and precision. For example, Texas Instruments (TI) proposed the patent (U.S. Pat. No. 6,832,171) to estimate the battery capacity and the aging of the battery as follows:
Please refer to FIG. 1, a battery pack 1 includes batteries 10, a battery management system 20, a charge and discharge transistor 40 and a resistor 50. The battery pack provides current to the load 30 (system load) and the current is ILOAD. The battery supply voltage between the positive electrode and the negative electrode of the battery is VBAT. The current ILOAD passes through the resistor 50 and the current ILOAD can be obtained by measuring the voltage across the resistor 50. With measuring the open circuit voltage VOC of the battery 10, the battery internal resistance DCIR can be calculated as DCIR=(VOC−VBAT)/ILOAD.
Please refer to FIG. 2. During the discharge, when the depth of discharge (DOD: depth of discharge) is from 0% to 80%, the battery internal resistances are recorded for every 10% DOD changes; when the depth of discharge (DOD: depth of discharge) is from 80% to 100%, the battery internal resistances are recorded for every 3.3% DOD changes, wherein OCV is the open circuit battery voltage and dV is the difference between the open circuit voltage and the battery supply voltage, that is, VOC−VBAT.
The battery internal resistance R values are stored in two types table: (1) Ra Table: no temperature correction of the R value, (2) Rb Table: R-value after temperature correction.
This battery internal resistance measurement is based on the system load current ILOAD, but system load currents ILOAD are instable and variable on each measurement point. The battery internal resistance data obtained by this method are complex and difficult to be analyzed.
In view of the above issues, the present invention provides a battery capacity measurement by the battery internal resistance of a constant discharge circuit.